将状态变量传递给父视图

Question

我有以下代码：

struct BookView: View {
    
    @State var title = ""
    @State var author = ""

    var body: some View {
        TextField("Title", text: $title)
        TextField("Author", text: $author)
    }
}

struct MainView: View {
    @State private var presentNewBook: Bool = false
    var body: some View {
        NavigationView {
            // ... some button that toggles presentNewBook
        }.sheet(isPresented: $presentNewBook) {
        let view = BookView()
        view.toolbar {
            ToolbarItem(placement: principal) {
                TextField("Title", text: view.$title)
            }
        }
    }
    }
}

这会编译，但在运行时给我以下错误：

Accessing State's value outside of being installed on a View. This will result in a constant Binding of the initial value and will not update.

如何将状态变量传递给其他外部视图？我不能在BookView 上使用ObservableObject，因为这需要我将它从struct 更改为class

Answer 1

您也可以像这样在视图之间传递状态变量：

let view = BookView(title: "foobar")
view.toolbar {
     ToolbarItem(placement: principal) {
         TextField("Title", text: view.$title)
     }
}

那么，BookView内部：

@State var title: String

init(title: String) {
    _title = State(initialValue: title)
}

来源：How could I initialize the @State variable in the init function in SwiftUI?

Answer 2

一般来说，您的状态应始终在视图层次结构的上层拥有。尝试从父级访问子级状态是一种反模式。

一种选择是使用@Bindings 将值传递给子视图：

struct BookView: View {
    
    @Binding var title : String
    @Binding var author : String
    
    var body: some View {
        TextField("Title", text: $title)
        TextField("Author", text: $author)
    }
}

struct ContentView: View {
    @State private var presentNewBook: Bool = false
    
    @State private var title = ""
    @State private var author = ""
    
    var body: some View {
        NavigationView {
            VStack {
                Text("Title: \(title)")
                Text("Author: \(author)")
                Button("Open") {
                    presentNewBook = true
                }
            }
        }.sheet(isPresented: $presentNewBook) {
            BookView(title: $title, author: $author)
        }
    }
}

另一种可能性是使用ObservableObject：

class BookState : ObservableObject {
    @Published var title = ""
    @Published var author = ""
}

struct BookView: View {
    
    @ObservedObject var bookState : BookState
    
    var body: some View {
        TextField("Title", text: $bookState.title)
        TextField("Author", text: $bookState.author)
    }
}

struct ContentView: View {
    @State private var presentNewBook: Bool = false
    @StateObject private var bookState = BookState()
    
    var body: some View {
        NavigationView {
            VStack {
                Text("Title: \(bookState.title)")
                Text("Author: \(bookState.author)")
                Button("Open") {
                    presentNewBook = true
                }
            }
        }.sheet(isPresented: $presentNewBook) {
            BookView(bookState: bookState)
        }
    }
}

我对您的示例视图进行了一些更改，因为对我来说结构不清楚，但在父级别拥有状态的概念是重要的元素。